fix: preserve json structure when patching within this library

src/trace/patch-console.spec.ts

@@ -41,55 +41,114 @@ describe("patchConsole", () => {
     unpatchConsole(cnsole as any);
   });
 
-  it("injects trace context into log messages", () => {
+  it.each([
+    { method: "log", mock: () => log },
+    { method: "info", mock: () => info },
+    { method: "debug", mock: () => debug },
+    { method: "error", mock: () => error },
+    { method: "warn", mock: () => warn },
+    { method: "trace", mock: () => trace },
+  ] as const)("injects trace context into $method messages", ({ method, mock }) => {
+    patchConsole(cnsole as any, contextService);
+    cnsole[method]("Hello");
+    expect(mock()).toHaveBeenCalledWith("[dd.trace_id=123456 dd.span_id=78910] Hello");
+  });
+
+  it("doesn't inject trace context when none is present", () => {
+    contextService["rootTraceContext"] = undefined as any;
     patchConsole(cnsole as any, contextService);
     cnsole.log("Hello");
-    expect(log).toHaveBeenCalledWith("[dd.trace_id=123456 dd.span_id=78910] Hello");
+    expect(log).toHaveBeenCalledWith("Hello");
   });
-  it("injects trace context into debug messages", () => {
+  it("injects trace context into empty message", () => {
     patchConsole(cnsole as any, contextService);
-    cnsole.info("Hello");
-    expect(info).toHaveBeenCalledWith("[dd.trace_id=123456 dd.span_id=78910] Hello");
+    cnsole.log();
+    expect(log).toHaveBeenCalledWith("[dd.trace_id=123456 dd.span_id=78910]");
   });
-  it("injects trace context into debug messages", () => {
+  it("injects trace context into JSON-style log by adding dd property", () => {
     patchConsole(cnsole as any, contextService);
-    cnsole.debug("Hello");
-    expect(debug).toHaveBeenCalledWith("[dd.trace_id=123456 dd.span_id=78910] Hello");
+
+    cnsole.log({ objectKey: "objectValue", otherObjectKey: "otherObjectValue" });
+    expect(log).toHaveBeenCalledWith({
+      objectKey: "objectValue",
+      otherObjectKey: "otherObjectValue",
+      dd: {
+        trace_id: "123456",
+        span_id: "78910",
+      },
+    });
   });
-  it("injects trace context into error messages", () => {
+
+  it.each([
+    { name: "array", value: [1, 2, 3], expected: "[dd.trace_id=123456 dd.span_id=78910] 1,2,3" },
+    { name: "null", value: null, expected: "[dd.trace_id=123456 dd.span_id=78910] null" },
+    { name: "number", value: 42, expected: "[dd.trace_id=123456 dd.span_id=78910] 42" },
+    { name: "undefined", value: undefined, expected: "[dd.trace_id=123456 dd.span_id=78910] undefined" },
+  ])("injects trace context as string prefix for $name", ({ value, expected }) => {
     patchConsole(cnsole as any, contextService);
-    cnsole.error("Hello");
-    expect(error).toHaveBeenCalledWith("[dd.trace_id=123456 dd.span_id=78910] Hello");
+    cnsole.log(value);
+    expect(log).toHaveBeenCalledWith(expected);
   });
-  it("injects trace context into error messages", () => {
+
+  it("injects trace context as string prefix when multiple arguments provided", () => {
     patchConsole(cnsole as any, contextService);
-    cnsole.warn("Hello");
-    expect(warn).toHaveBeenCalledWith("[dd.trace_id=123456 dd.span_id=78910] Hello");
+
+    cnsole.log({ key: "value" }, "extra arg");
+    expect(log).toHaveBeenCalledWith("[dd.trace_id=123456 dd.span_id=78910] [object Object]", "extra arg");
   });
-  it("injects trace context into error messages", () => {
+
+  it("injects trace context as string prefix for class instances", () => {
     patchConsole(cnsole as any, contextService);
-    cnsole.trace("Hello");
-    expect(trace).toHaveBeenCalledWith("[dd.trace_id=123456 dd.span_id=78910] Hello");
+
+    class MyClass {
+      value = "test";
+    }
+    const instance = new MyClass();
+    cnsole.log(instance);
+    expect(log).toHaveBeenCalledWith("[dd.trace_id=123456 dd.span_id=78910] [object Object]");
   });
 
-  it("doesn't inject trace context when none is present", () => {
-    contextService["rootTraceContext"] = undefined as any;
+  it("injects trace context into JSON-style log created with Object.create(null)", () => {
     patchConsole(cnsole as any, contextService);
-    cnsole.log("Hello");
-    expect(log).toHaveBeenCalledWith("Hello");
+
+    const obj = Object.create(null);
+    obj.message = "test";
+    cnsole.log(obj);
+    expect(log).toHaveBeenCalledWith({
+      message: "test",
+      dd: {
+        trace_id: "123456",
+        span_id: "78910",
+      },
+    });
   });
-  it("injects trace context into empty message", () => {
+
+  it("preserves nested objects in JSON format", () => {
     patchConsole(cnsole as any, contextService);
-    cnsole.log();
-    expect(log).toHaveBeenCalledWith("[dd.trace_id=123456 dd.span_id=78910]");
+
+    cnsole.log({ level: "info", nested: { foo: "bar" } });
+    expect(log).toHaveBeenCalledWith({
+      level: "info",
+      nested: { foo: "bar" },
+      dd: {
+        trace_id: "123456",
+        span_id: "78910",
+      },
+    });
   });
-  it("injects trace context into logged object message", () => {
+
+  it("merges trace context with existing dd property", () => {
     patchConsole(cnsole as any, contextService);
 
-    cnsole.log({ objectKey: "objectValue", otherObjectKey: "otherObjectValue" });
-    expect(log).toHaveBeenCalledWith(
-      "[dd.trace_id=123456 dd.span_id=78910] { objectKey: 'objectValue', otherObjectKey: 'otherObjectValue' }",
-    );
+    cnsole.log({ message: "test", dd: { existing: "value" } });
+    expect(log).toHaveBeenCalledWith({
+      message: "test",
+      dd: {
+        existing: "value",
+        trace_id: "123456",
+        span_id: "78910",
+      },
+    });
   });
   it("leaves empty message unmodified when there is no trace context", () => {
     contextService["rootTraceContext"] = undefined as any;

src/trace/patch-console.ts

@@ -1,5 +1,4 @@
 import * as shimmer from "shimmer";
-import { inspect } from "util";
 
 type Console = typeof console;
 
@@ -10,6 +9,37 @@ import { TraceContextService } from "./trace-context-service";
 
 type LogMethod = "log" | "info" | "debug" | "error" | "warn" | "trace";
 
+/**
+ * Checks if a value is a JSON-style structured log (plain object).
+ * When true, trace context will be injected as a `dd` property to preserve JSON format.
+ * When false, trace context will be prepended as a string prefix.
+ */
+function isJsonStyleLog(value: unknown): value is Record<string, unknown> {
+  if (value === null || typeof value !== "object") {
+    return false;
+  }
+  if (Array.isArray(value)) {
+    return false;
+  }
+  const proto = Object.getPrototypeOf(value);
+  return proto === null || proto === Object.prototype;
+}
+
+/**
+ * Checks if a value is a plain object (not null, not an array).
+ */
+function isPlainObject(value: unknown): value is Record<string, unknown> {
+  return value !== null && typeof value === "object" && !Array.isArray(value);
+}
+
+/**
+ * Extracts the existing `dd` property from a log object if it's a plain object.
+ * Returns an empty object if `dd` is missing or not a plain object.
+ */
+function getExistingDdContext(logObject: Record<string, unknown>): Record<string, unknown> {
+  return isPlainObject(logObject.dd) ? logObject.dd : {};
+}
+
 /**
  * Patches console output to include DataDog's trace context.
  * @param contextService Provides up to date tracing context.
@@ -51,27 +81,33 @@ function patchMethod(mod: wrappedConsole, method: LogMethod, contextService: Tra
       }
       isLogging = true;
 
-      let prefix = "";
       const oldLogLevel = getLogLevel();
       setLogLevel(LogLevel.NONE);
       try {
         const context = contextService.currentTraceContext;
         if (context !== null) {
           const traceId = context.toTraceId();
-          const parentId = context.toSpanId();
-          prefix = `[dd.trace_id=${traceId} dd.span_id=${parentId}]`;
+          const spanId = context.toSpanId();
+
           if (arguments.length === 0) {
+            // No arguments: emit just the trace context prefix
             arguments.length = 1;
-            arguments[0] = prefix;
+            arguments[0] = `[dd.trace_id=${traceId} dd.span_id=${spanId}]`;
+          } else if (arguments.length === 1 && isJsonStyleLog(arguments[0])) {
+            // Single plain object: inject dd property to preserve JSON format
+            arguments[0] = {
+              ...arguments[0],
+              dd: {
+                ...getExistingDdContext(arguments[0]),
+                // Overwrite trace_id and span_id to ensure we have the latest values
+                trace_id: traceId,
+                span_id: spanId,
+              },
+            };
           } else {
-            let logContent = arguments[0];
-
-            // If what's being logged is not a string, use util.inspect to get a str representation
-            if (typeof logContent !== "string") {
-              logContent = inspect(logContent);
-            }
-
-            arguments[0] = `${prefix} ${logContent}`;
+            // String or multiple arguments: use string prefix
+            const prefix = `[dd.trace_id=${traceId} dd.span_id=${spanId}]`;
+            arguments[0] = `${prefix} ${arguments[0]}`;
           }
         }
       } catch (error) {